I. Field of the Invention
The present invention relates to a device for machining the crank pins of a crankshaft.
II. Description of the Prior Art
Crankshafts of the type used in internal combustion reciprocal piston engines include at least two main bearings which are rotatably mounted to the engine crankcase and a plurality of crank pins to which the piston connecting rods are rotatably attached. Furthermore, these crank pins have an axis substantially parallel to, but radially spaced from, the rotational axis of the crankshaft. Consequently, the crank pins move cyclically during rotation of the crankshaft.
Both the main bearings and the crank pins for the crankshaft must be machined in order to form a bearing surface. Machining the main bearings is straightforward since the crankshaft is merely rotatably driven about its axis and the main bearings machined by turning, grinding, or the like.
Conversely, machining the crank pins to form the required bearing surface presents a more difficult problem since the axis of the crank pins are radially offset from the rotational axis of the crankshaft. There have been a number of previously known devices and methods for machining the crank pins.
One previously known method for machining the crank pins is to rotatably drive the crankshaft about the crank pins axis and then machine the crank pins in any conventional fashion. This previously known method, however, is undesireable since the crankshaft must be rotatably driven around each different crank pin axis. As such, multiple sequential machining operations must be performed on the crankshaft which increases the overall machining costs and time for the crankshaft.
There have, however, been a number of previously known machines which are designed to simultaneously machine all of the crank pins for the crankshaft. These previously known machines typically comprise a first master shaft having a plurality of offset crank pins which correspond to the crank pins of the crankshaft which is to be machined. The master shaft is rotatably mounted to a housing while another substantially identical master shaft is rotatably mounted to one end of a cradle. The other end of the cradle is pivotally mounted around the rotational axis of the first master shaft and movable between a first and a second position. Since the master shafts are substantially identical to each other, each crank pin on the first master shaft has a corresponding crank pin on the second master shaft which is both radially offset and angularly positioned the same.
A plate or girt is then rotatably mounted to each of corresponding crank pins on both master shafts so that, as the master shafts and the crankshaft are rotatably driven in synchronism with each other, each girt replicates the cyclical movement for its associated crank pin on the crankshaft. A tool bit mounted to each girt engages its associated crank pin on the crankshaft as the cradle is pivotted from its first and to its second position to thereby machine the crankshaft crank pins. Conventionally, the tool engages the crank pin diametrically as the cradle is pivoted from its first and to its second position.
One disadvantage of these previously known crankshaft machining devices is that all crank pins are machined simultaneously with each other. As such, the tools as well as the crankshaft undergo a tremendous tool load or tool pressure during the machining operation. This high tool load not only causes heating and occasional distortion of the crankshaft, but also results in a relatively short tool life for the cutting tools. As such, the tools must be frequently replaced thereby resulting in down time for the machine.